1. Field of the Invention
This invention pertains to a cable transmission system, such as commonly employed in communicating seismic data from a plurality of localized data gathering stations or units to a control or master station, and more particularly to testing the local data units for the presence of possible environmental water leakage.
2. Description of the Prior Art
A seismic data gathering system commonly comprises a plurality of local data stations or units spaced apart along a line, or in a predetermined grid pattern. The locations are determined to give satisfactory terrain coverage so that the data collection from seismic reflections will be more or less uniform and so that there will not be any appreciable data skips.
Sometimes this results in the local stations, commonly referred to as remote data units (RDU's), being located in low lying areas subject to occasional flooding and even in swampy conditions where water stands virtually all of the time.
Each data unit receives local data via one or more data measuring devices, commonly one or more geophones. More commonly, an array of geophones provides in time multiplex fashion a plurality of local data measurements. In more sophisticated systems, each data measuring input to the local data unit is a composite from a plurality of individual geophone connections.
A geophone connection is normally made via a coil in the geophone which is connected into an input stage of the electronics at the data unit. The electronics at the data unit comprises a plurality of networks but generally includes preamplifiers, filters and amplifiers as well as the communication components necessary to provide communication with the master or control station. In addition, the local electronics at an RDU permits data multiplexing with other data information originating at other RDU's. Also, control components are included in the RDU to permit reception of control signals for providing operation in accordance with the system and for providing testing.
The electronics of each RDU is generally housed in a common case, which is normally painted or anodized metal or plastic. Although the electronics within the case are connected to a circuit common or ground within the case, such common is isolated from earth ground unless an earth ground connection is also provided. Because of the temporary nature of setting the RDU's in place and the difficult locations that are sometimes involved, it is not readily possible to provide a local ground connection. In fact, because of difficult terrains encountered in many installations and because of the inconvenience to the crew of having to make a local ground connection at the RDU's, a local earth ground connection is not normally provided and, hence, the circuit common at each RDU "floats" without being connected to earth ground.
The individual data measurement units, commonly geophones, are desirable watertight. Further, the RDU's are desirable watertight. However, it is well known that leakage does occur and when it occurs there is generally established a conductive path to ground that causes depreciation of the developed signals. This depreciation or decrease in the signal strength is sometimes not readily ascertainable since the impedance path to ground or earth which causes the signal depreciation is sometimes a sufficiently high impedance path that the data signals are not totally lost but only lowered in amplitude. Obviously, a loss in amplitude of a seismic measurement has adverse interpretation consequences for analyzing the data measurements. Furthermore, when an extraneous path is provided into the system, the possibility also exists for a hum or spurious signal to be superimposed upon the data. In many areas, a 60 Hz signal is superimposed upon the data, resulting in a further decrease or depreciation of the measured data signals.
When water does leak into a geophone or the compartment housing the RDU, there is established this aforementioned impedance path to earth, which may be characterized as an equivalent circuit that is somewhat resistive and somewhat capacitive. Also, the path may be quite severe or might be slight, in which case it would be characterized as a very high impedance path. In either event, it is a problem to the development of reliable data from the installation.
In prior art cable transmission systems, it has been incumbent upon the crews laying the cable to carefully inspect for leakage. This condition cannot be reliably remotely controlled or monitored in an acceptable manner. Before the present invention, the possible existence of leakage was a condition that just had to be tolerated. If the condition was so bad that the data was obviously adversely affected, then the suspect components were replaced and the data redeveloped, all at great expense and loss of time.
Therefore, it is a feature of the present invention to provide improved leakage testing apparatus for remotely determining the presence thereof of water leakage at the RDU and/or at the individual data measuring devices which are attached to the RDU, namely, geophones.
It is another feature of the present invention to provide an improved testing of a cable transmission system with intermediate data units therealong and having a master control station where testing control signals and ground connection therefor are all by way of connection through the master control station.
It is still another feature of the present invention to provide an improved cable data transmission system in which it is unnecessary to establish an earth ground at the individual remote data units since it is determined before operation that there is virtually no leakage, data-impairment connection to ground.